1. FIELD OF THE INVENTION
This invention relates to a method and apparatus for estimating the life expectancy of mechanical structures to which a repeated load is applied, and more particularly it is concerned with a method and apparatus for estimating the life expectancy of mechanical structures to which a repeated or fluctuating load is applied in a high temperature atmosphere.
2. DESCRIPTION OF THE PRIOR ART
Mechanical structures which are subjected to a repeated load in a high temperature atmosphere include, inter alia, turbines of electric power plants. In a turbine of an electric power plant, structural members of a turbine suffer damage due to thermal fatigue and creep, when the turbine is started and shut down repeatedly or the load applied thereto undergoes fluctuations. The result of this would be that the structural members might develop crack formation due to accumulated damage caused by the fatigue and creep, thereby causing a reduction to occur in the strength of the structure. In this case, there would be the risk that, unless an estimation was made of the life expectancy of the structure from the standpoint of its strength, rupture of equipment might occur and cause an accident on a large scale to take place in the electric power plant. In this connection, it is of particular importance that estimation of the life expectancy of the turbine casing, rotor and various types of valves be made from the standpoint of their strength.
Heretofore, it has been the usual practice, in designing this type of equipment, to set a high factor of safety based on the creep strength of the material used, thereby increasing reliability. Thus, estimation has rarely been made of the life expectancy of the equipment of the electric power plant. As it now stands, however, about one half the oil-burning electric power plants now in operation have passed the limit of their designed useful life, and the time has come to determine whether they can be left in service or replaced. Thus, it is now earnestly desired that a reliable method of estimating the life expectancy of mechanical structures be established.
Meanwhile, a change in demand for electric power has made it necessary to operate electric power plants, which were designed to meet the requirement of a base load, in such a manner as to start and shut down to cope with changes in load. When the operating conditions become severe, it is presumed that the useful life of the electric power plants naturally becomes short. Thus, it is quite important to estimate the life expectancy of such plants.
If it is possible to determine by non-destructive tests the damage suffered by the turbine casing, main valve and regulating valves to enable the life expectancy to be estimated, the reliability of a plant would be greatly improved. However, no proposals have ever been made to provide a method or an apparatus which would admirably suit this purpose.
With regard to the turbine rotor, however, system proposals have been made, as disclosed in Japanese Patent Examined Publication Nos. 21169/80 and 25842/83, for example, to sense the temperature of steam and gas in the vicinity of the rotor and the temperature on the inner surface of the casing. Based on the values of the sensed temperatures the system calculates 12 thermal stresses which would be applied to the rotor and controls the operation of the turbine based on the rate of change and the absolute value of the thermal stresses, to thereby avoid the development of excessively high thermal strain in the rotor.
The system referred to hereinabove is intended to avoid the development of excessively high thermal strain in the turbine rotor and to prevent the service life of the rotor from becoming shorter than its designed useful life, and is unable to estimate the life expectancy of the rotor. Particularly, in those plants which have been operated under severe conditions, it is impossible for this system to estimate the life expectancy of the rotor therein.
As a code for estimating the life expectancy of a steam turbine rotor, "SAFER" is known which is based on linear destruction dynamics (Research Report No. 283021, reported by Electric Power Central Research Institute, December 1983).
An electrochemical method has been applied to the art of non-destructive diagnosis of deterioration with respect to time in steam turbines as reported in On-the-Site Technology of Electricity, Vol. 23, No. 261.
The problem with these proposals of the prior art is that, although they may produce results which are somewhat reliable under a certain condition, they produce conflicting results and are misleading under other conditions.